Language -- a signature of our species -- derives its power from its links to cognition. In the first investigation of its kind, this proposal will identify the neural correlates underlying infants' establishment of this interface between language and cognition in their first six months. The proposal builds upon two strong foundations: a) striking new behavioral evidence from our laboratory documenting that long before infants begin to speak, they have begun to link language and cognition, and b) recent neuroscientific evidence documenting that EEG, and neural oscillatory activity in particular, reveals the neural processes underlying language processing and its effects on cognition in infancy. In behavioral work, we have shown that by 3 months, simply listening to human language supports infant cognition, that at 3 and 4 months, vocalizations of nonhuman primates confer the same cognitive advantage as human vocalizations, that by 6 months, infants have narrowed the link specifically to human language, and that between 4 and 6 months, infants' exposure to language and other signals is crucial as they tune this link. The current proposal is designed to identify, for the first time, infants' neural responses they listen to sounds, including human vocalizations and lemur vocalizations. Study 1 is designed to identify 4- and 6-month-old infants' neural responses to vocalizations produced by humans and non-human primates, as well as carefully-selected control signals including backward speech. Studies 2 and 3 are designed to uncover whether (and how) infants' neural responses to these signals is affected by exposure between 4 and 6 months, a critical period during which infants tune the behavioral link specifically to human vocalizations. By weaving our expertise in infant behavioral research with state-of-the-art developmental neuroscience techniques, this proposal will illuminate how infant brain systems interact in infants' first months, as they forge their earliest links between language and cognition, and how these links are shaped by experience. The proposed project broadens significantly the empirical and theoretical foundations of current research. By identifying the neural correlates underlying infants' surprisingly early behavioral links between language and cognition, it will illuminate how infant brain systems interact from the start and how they are shaped by infants' experience. This work will shed light on the neural foundations that underlie the origin and evolution of infants' earliest links between conceptual and linguistic development. This new evidence will provide a foundation for identifying very young infants at risk for language delay or impairment, and will serve as a springboard for designing targeted, neurodevelopmentally-inspired interventions to bolster language and cognitive advances in the first months of life.